Outline

Background: The increased effectiveness of highly conformal ion beam therapy requires a higher precision on the dose monitoring and location than in conventional radiation therapy. To monitor in real time the longitudinal position of the Bragg peak we proposed a novel non invasive technique (Testa E, et al. Appl Phys Lett. 2008;93. 093506) that exploits the detection of prompt gamma-rays issued from nuclear fragmentation (Gunzert-Marx K, et al. New J Phys. 2008;10. 075003). In our first experiment (Testa E, et al. Appl Phys Lett. 2008;93. 093506) with a 73 MeV/u 13C6+ ion beam we showed that i) the ion depth-dose profile is correlated with the prompt-gamma-ray profile and that ii) the use of the time-of-flight technique allows us to discriminate between direct gammas and neutron-induced background. Therefore, a compact device, with large detection solid angle and minimum shielding, could provide counting rates high enough for real-time control of the Bragg peak position. This represents a considerable improvement of the passive neutron-shielded camera previously developed by Min et al (Min CH, et al. Appl Phys Lett. 2006;89. 183517).

Material and methods: A new experiment was performed at GANIL with a 95 MeV/u 12C6+ ion beam. The carbon beam was stopped in a PMMA target formed by stacking 2 mm-thick plates. To determine the gamma-rays source points emitted by fragment de-excitation we used a collimated detector placed at 90Â° with respect to the beam direction and aiming at the target. The gamma-ray profile and therefore the Bragg peak localization was obtained by translating the target along the beam axis relative to the collimator.

Results: This communication shows how target variations (both in density and volume) influence the photon production.

First, we have inserted lighter and heavier material in the stacked PMMA target. We observed that the density of the material traversed by the beam strongly affects the photon production rate and therefore the gamma profile correlated with the ion depth-dose profile.

Then we changed the shape and the size of the target to study the influence of the target volume on the photon scattering yield. As expected, the signal-to-background ratio decreases when the target volume increases due to the higher probability of scattering.

Conclusion: The proposed technique remains valid with patient-like target volumes, and the gamma profile is in any case correlated with the ion depth of penetration. Moreover, information on the density can be extracted from the gamma yields.